1. Field of the Invention
The present invention relates to an electrodeless discharge lamp that emits light under an electromagnetic field generated by an induction coil arranged in a reentrant portion of a bulb.
2. Description of the Related Art
Recent years have seen a widespread use of fluorescent lamps with higher efficiency and longer life than electric bulb from the viewpoint of global environmental protection. Further, in addition to conventional fluorescent lamps comprising electrodes, electrodeless lamps are under research. Having no electrodes—a factor restricting the life of conventional lamps with electrodes, electrodeless lamp has the advantage that its life is several times longer than that of lamps with electrode, thus holding promise for future widespread use.
Such an electrodeless lamp produces a discharge plasma with a high-frequency electromagnetic field generated by an induction coil arranged in a reentrant portion of a bulb. Having a shape of solenoid of a finite length, the induction coil forms an open magnetic circuit, causing the magnetic field to leak out of the induction coil.
To prevent the magnetic field from leaking out of the induction coil, Japanese Patent Application Laid-Open Publication No. 1995-262972 teaches using a short-circuited metal ring shown in FIG. 13. According to the teaching, a short-circuited metal ring 9 is arranged on the outer perimeter surface of a bulb 1, and as substantially all magnetic fields generated from the induction coil 3 induce current within the metal ring 9, magnetic flux leaking out of the lamp is suppressed, thus suppressing fixture interference. This ensures that there are substantially no changes between when the lamp is attached and when it is not attached to metallic fixture (see, e.g., Japanese Patent Application Laid-Open Publication No. 1995-262972).
The present inventors have found that when an electrodeless lamp operates on power at a relatively low driving frequency (e.g., 1 MHz or less), provision of a short-circuited metal ring as disclosed in Japanese Patent Application Laid-Open Publication No. 1995-262972 near the bulb will considerably reduce the starting pulse voltage generated in the induction coil during lamp startup, making it difficult, in the worst case, to start the lamp and maintain it lit. The present inventors have also discovered that, even in the absence of such a metal ring, a similar problem will arise if the electrodeless lamp is used as attached to metallic lighting fixture, etc.
Thus, in the presence of a metal ring such as short-circuited metal ring or lighting fixture near the electrodeless lamp, the starting pulse voltage generated in the induction coil will decline considerably, making it difficult, in the worst case, to start the lamp and maintain it lit. In the present specification, this phenomenon is referred to as “fixture interference.” The reason why fixture interference occurs is deemed to be attributable to mutual induction occurring between the induction coil and the metal ring as a result of crossing of leaked magnetic field with the metal ring. That is, if the winding of the induction coil is assumed to be the primary winding of the coil magnetic core, the metal ring such as a short-circuited ring or a lighting fixture is equivalent to the secondary winding of the coil magnetic core. If the resistance value of the metal ring is sufficiently reduced to minimize losses in the metal ring, the Q value of the induction coil will decline considerably. On the other hand, if the distance is close between the metal portion of the lighting fixture and the induction coil, mutual induction will unavoidably increase, reducing the Q value of the induction coil. This results in difficulties in generation of the starting voltage at both ends of the induction coil—a voltage required to initiate electric discharge, possibly deteriorating the startability of the lamp.
Thus, depending on the magnitude of mutual inductance in fixture interference, the starting pulse voltage, generated in the induction coil during lamp startup, will decrease considerably, making it difficult, in the worst case, to start the lamp and keep it lighting.
As described earlier, this problem of lamp startability is prominent if the high-frequency power used for discharge is low in frequency (driving frequency). The reason is that electric discharge readily occurs at a high driving frequency, making decline in Q value of the induction coil trivial. Currently under research is further reduction in frequency of high-frequency power used for electric discharge. For this reason, the demands are high for development of a technology for avoiding decline in Q value of the induction coil caused, for example, by fixture interference.